Method of producing a graphene film as transparent and electrically conducting material

ABSTRACT

A transparent and conductive film comprising at least one network of graphene flakes is described herein. This film may further comprise an interpenetrating network of other nanostructures, a polymer and/or a functionalization agent(s). A method of fabricating the above device is also described, and may comprise depositing graphene flakes in solution and evaporating solvent therefrom.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of prior U.S. application Ser. No.11/563,623, filed on Nov. 27, 2006 now U.S. Pat. No. 7,499,133 andentitled “GRAPHENE FILM AS TRANSPARENT AND ELECTRICALLY CONDUCTINGMATERIAL,” which claims priority to U.S. Provisional Application No.60/812,977, filed Jun. 13, 2006 and entitled “GRAPHENE FILM ASTRANSPARENT AND ELECTRICALLY CONDUCTING MATERIAL.” Each of the foregoingapplications is commonly assigned to the assignee of the presentinvention and is hereby incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates generally to transparent and conductingfilms, and more specifically to transparent and conducting filmscomprising at least one network of graphene.

BACKGROUND OF THE INVENTION

Many modern and/or emerging applications require at least one deviceelectrode that has not only high electrical conductivity, but highoptical transparency as well. Such applications include, but are notlimited to, touch screens (e.g., analog, resistive, improved analog, X/Ymatrix, capacitive), flexible displays (e.g., electro-phoretics,electro-luminescence, electrochromatic), rigid displays (e.g., liquidcrystal (LCD), plasma (PDP), organic light emitting diode (LED)), solarcells (e.g., silicon (amorphous, protocrystalline, nanocrystalline),cadmium telluride (CdTe), copper indium gallium selenide (CIGS), copperindium selenide (CIS), gallium arsenide (GaAs), light absorbing dyes,quantum dots, organic semiconductors (e.g., polymers, small-moleculecompounds)), fiber-optic communications (e.g., electro-optic andopto-electric modulators) and microfluidics (e.g. electrowetting ondielectric (EWOD)). As used herein, a layer of material or a sequence ofseveral layers of different materials is said to be “transparent” whenthe layer or layers permit at least 50% of the ambient electromagneticradiation in relevant wavelengths to be transmitted through the layer orlayers. Similarly, layers which permit some but less than 50%transmission of ambient electromagnetic radiation in relevantwavelengths are said to be “semi-transparent.”

Currently, the most common transparent electrodes are transparentconducting oxides (TCOs), specifically indium-tin-oxide (ITO) on glass.However, ITO can be an inadequate solution for many of theabove-mentioned applications (e.g., due to its relatively brittle natureand correspondingly inferior flexibility and abrasion resistance), andthe indium component of ITO is rapidly becoming a scarce commodity.Additionally, ITO deposition usually requires expensive,high-temperature sputtering, which can be incompatible with many deviceprocess flows. Hence, more robust and abundant transparent conductormaterials are being explored.

Single-walled carbon nanotubes (SWNTs) have attracted a great deal ofinterest, due to their unique mechanical and electrical properties.Highly conductive SWNT networks having a dc conductivity of at leastabout 4000 Siemens/cm and methods of fabricating these together with theink material that is used for the fabrication have been described in theliterature. However, although nanotube networks fabricated to date areboth conducting and transparent, they have not been able to achieve theright combination of sheet conductance and transparency to becompetitive with currently used materials such as indium-tin-oxide(ITO). (L. Hu et al Nano Letters 4, 2513 (2004)) N. P. Armitage, J-C PGabriel and G. Gruner, “Langmuir-Blodgett Nanotube Films”, J. Appl.Phys. Lett, 95, 6, 3228-3330 (2003)).

SUMMARY

The present invention relates to an interconnected network, or filmcomprised of finite sized graphene sheets, called “flakes”, as atransparent and electrically conducting material. One embodiment of theinvention is a transparent and electrically conducting network comprisedof multiple flakes of graphene. Another embodiment of the invention is atransparent and electrically conductive network comprised of multipleflakes of graphene in combination with other nanostructured materials,such as carbon nanotubes. In another embodiment of the invention, atransparent and electrically conductive network is comprised offunctionalized graphene flakes. A further embodiment of the invention isfilms comprised of at least one layer of graphene flakes and at leastone layer of another material. In another embodiment, a transparent andelectrically conductive network is made of graphene-polymer composites.In a sixth embodiment, new and improved techniques are used to depositegraphene films in different ways onto different substrates. A finalembodiment of the invention is new and improved devices based onintegration of graphene films.

Graphene in this invention is defined as single or multiple layers ofgraphene. Novoselov, K. S. et. al. PNAS, Vol. 102, No. 30, 2005,Novoselov, K. S. et. al. Science, Vol 306, 2004. Graphene films compriseat least one network of graphene “flakes”. Graphene “flakes” arefinite-area graphene constructions.

Other features and advantages of the invention will be apparent from theaccompanying drawings and from the detailed description. One or more ofthe above-disclosed embodiments, in addition to certain alternatives,are provided in further detail below with reference to the attachedfigures. The invention is not limited to any particular embodimentdisclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is better understood from reading the following detaileddescription of the preferred embodiments, with reference to theaccompanying figures in which:

FIG. 1. is an illustration of graphene flakes on a substrate.

FIG. 2. is an illustration of a graphene flake—carbon nanotube network.

FIG. 3. is an SEM image of a graphene flake on a substrate.

FIG. 4. is an SEM image of a Graphene flake/CNT network.

FIG. 5. is a graph of the optical transparency of a graphene networkwith a 1 Mohm sheet resistance.

Features, elements, and aspects of the invention that are referenced bythe same numerals in different figures represent the same, equivalent,or similar features, elements, or aspects in accordance with one or moreembodiments of the system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention comprises transparent and conductive materials comprisinggraphene flakes and devices incorporating such materials.

Graphene is a single-atomic-layer of graphite and as such is expected tobe a zero-gap semiconductor. Although it is only one atom thick andunprotected from the immediate environment, graphene exhibits highcrystal quality and ballistic transport at submicron distances.Moreover, graphene can be light, highly flexible and mechanically strong(resisting tearing by AFM tips), and the material's dense atomicstructure should make it impermeable to gases.

A SWNT can be conceptualized by wrapping a graphene layer into aseamless cylinder. It is recognized that the SWNT's electronicproperties stem from the properties of the graphene layer and thatlayer's unusual band structure (only two bands crossing at the Fermilevel). Thus, it is anticipated that most of the electronic propertiesof SWNTs are shared by other low-dimensional graphitic structures.

First Embodiment Films Comprising Graphene Flakes

A first embodiment of the invention is a film formed by aninterconnected network of graphene flakes so that there is at least oneconduction path across the film.

Figure one illustrates graphene flakes on a substrate, and figure threeprovides an SEM image of a graphene flake on a substrate.

First, commercially available graphene flakes are obtained. The grapheneflakes can be treated depending on the desired transparency andconductivity of the resulting films. Examples of preparation steps thatcan be used to tailor the properties of the resulting films include, butare not limited to, thinning the graphene flakes or doping the grapheneflakes. After the graphene flakes have been properly prepared, they aredissolved in solvents such as organic solvents such as dichlorobenzene,chloroform, or dimethylformamide. The solvent can include aqueousdispersions with a suitable surfactant as a solubilization agent. Thesolvent can also include other solubilization agents such as DNA orpolymers. In a preferred embodiment, the solution is then sonicated fora period of time.

After being solubilized, the solution is purified to remove impuritiesand flakes that have undesirable sizes. An example of a suitablepurification method is centrifugation, which results in separation ofthe liquid containing soluble compounds and concentrated material at thebottom of the centrifuge.

The supernatant is then dispensed through a filter to form filmscomprised of networks of graphene flakes. An example of a suitablefilter is a porous alumina filter. A media such as water is then used towash away any remaining solvent or surfactant. The resulting films haveboth conductivity and transparency in the visible spectrum. For example,films achieved by practicing the invention were 50 kOhms/sq and 50%transparent.

EXAMPLE

Graphene films were fabricated by dispersing the graphene flakes (AsburyCarbon grade 3061) in an appropriate solvent, including dichlorobenzeneor surfactant aided dispersions in water. This may include Sodiumdodecyl sulfate in water. The graphene flakes, once dispersed in thesolvent, are sonicated by a probe sonicator, resulting in a blacksolution. This solution is centrifuged to remove larger flakes andimpurities. The supernatant is dispensed through a porous aluminafilter, and water is subsequently washed over it to remove any remainingsolvent/surfactant. The graphene films on the filter were transferprinted to a plastic substrate such as PET by the use of a PDMS stamp.The stamp is pressed against the film on the filter, and the film istransferred to the stamp. The stamp can then be pressed against aplastic substrate, and gently heated, to complete the transfer process.The graphene flake films were 50 kOhms/sq and 50% transparent.

Second Embodiment Interconnected Networks of Graphene Sheets and CarbonNanotubes

In another embodiment of the invention, graphene flakes are deposited incombination with other nanostructured materials, in particular carbonnanotubes, so that an interconnected graphene and nanotube layerprovides electrical conduction. Figure two is an illustration of agraphene flake-carbon nanotube network, and figure four is an SEM imageof a graphene flake-carbon nanotube network.

EXAMPLE

A graphene flake-carbon nanotube composite was fabricated by repeatingthe method for the fabrication of the graphene films, with the additionof carbon nanotubes to the solvent. An interpenetrating network ofgraphene flakes and carbon nanotubes also leads to a network that is 80%transparent and 2 kOhms/sq or 65% and 1 kOhm/sq, where the opticaltransmission spectra for a 1 kOhm/sq sample is shown in FIG. 5.

Third Embodiment Graphene Films that are Functionalized

In another embodiment of the invention, graphene films are comprised offunctionalized graphene flakes, or functionalized nanotubes incombination with graphene flakes, or functionalized nanotubes incombination with functionalized graphene flakes. Functionalizationinvolves attaching chemicals to nanostructured materials to change theproperties of the nanostructured materials such as the electron or holeconcentration or the mobility. As an example, the conductivity can beenhanced by attaching molecules to nanotubes or graphene flakes. Theeffect of such attachment is twofold. First, the carrier number (i.e.the electron or hole concentration) is changed. Second, the mobility ischanged through the potential the attached molecule creates. Generally,relatively strong binding to graphene is required in order to create astable structure, where the molecules are not removed by a liquid,mechanical effects and the like. Such strong binding however also leadsto a strong potential that decreases the mobility.

Examples of molecules that can be used to functionalize the grapheneflakes to tune the properties of films according to the presentinvention, include but are not limited to:

Type Examples Organic Compounds Tetracyanoquinodimathane TCNQTetracyanoethylene TCNE Polymers With Electron Acceptor PolyethyleneImine Groups Inorganic Species Bromine (Br) Chlorine (Cl) Iodine (I)Thionyl Chloride (SOCl₂) Sulphur Trioxide (SO₃) Nitrogen Dioxide (NO₂)Nitrosonium Tetrafluoroborate (NOBF₄) Nitronium Tetrafluoroborate(NO₂BF₄) Light Sensitive Materials Porphyrines

Fourth Embodiment Other Layers in Combination With Graphene Layers

In another embodiment of the invention, films are constructed that arecomprised of at least one layer of graphene flakes and at least onelayer of another material. Examples of other layers that might be usedinclude, but are not limited to: a polymer layer such as parylene, apoly-3,4-Ethylmethyldioxythophene, PEDOT; a light sensitive layercomprised of materials such aspoly((m-phenylenevinyle)-co-)2.3.diotyloxy-p-phenylene)), PmPV; apolymer layer with electron donating or withdrawing properties such aspolyethylene-imine (PEI); a layer comprised of materials withappropriate conducting and transparent properties and electron affinityof ionization potential; a layer of biomolecules such as bovine serumalbumin (BSA). The intercalation of the different layers of the filmscan be adjusted to optimize the desired properties of the films.

Multi-layered films can be fabricated by, for example, depositing a filmcontaining graphene flakes, then depositing a layer of differentmaterial, and then depositing an additional layer of film containinggraphene flakes. Known techniques can be used to deposit the layers ofmaterials. For example, after depositing the film containing grapheneflakes, a polymer layer could be deposited through spin coating toobtain a continuous layer of uniform thickness. In another embodiment, asolvent can be used to solvate both the graphene flakes and material inthe alternative layer, and both the graphene flakes and additionalmaterial could be sprayed down in combination.

Fifth Embodiment Graphene Sheet-Polymer Composites

In another embodiment, graphene flakes can also be combined with othermaterials, in particular polymers, to form an electrically conductingand optically transparent layer. The components of a composite film, caninclude (but are not limited to): conducting polymers such as PEDOT orpolyaniline; non-conducting polymers such as parylene; or functional(i.e. light sensitive) polymers such aspoly((m-phenylenevinyle)-co-2.3.diotyloxy-p-phenylene)), PmPV andpoly-ethylene-imine (PEI).

Sixth Embodiment Deposition of the Films from Solution

Another embodiment is deposition of the graphene films from solution.Although known methods can be used to deposit many films, new andimproved techniques are needed to deposit graphene films. Examples ofnew techniques designed specifically for deposition of graphene filmsinclude:

A. Spray Painting

The solution of dispersed graphene flakes can be spray painted onto aheated or non-heated substrate. The substrate may or may not befrequently rinsed during the spraying process to remove thesolubilization agent, or surfactant. The spraying solution may be of anyconcentration. The substrate surface may be functionalized to aid ingraphene adhesion. The network may be sprayed below the percolationdensity for flakes, at the percolation density for flakes, or above thepercolation density for flakes.

B. Drop Casting

A drop of the solution can be placed onto a substrate for a period oftime. The substrate may be functionalized to enhance graphene adhesion.The substrate with graphene may be rinsed by appropriate solvents.

C. Spin Coating

The solution can be spin coated along with an appropriate solvent toremove the surfactant simultaneously.

D. Vacuum Filtration

The solution can vacuum filtered through a porous membrane, with thegraphene film being deposited on top of the filter. The film can bewashed while on the filter with any of numerous liquids to removesurfactant, functionalization agents, or unwanted dirt.

E. Dip Coating

The substrate can be dipped into the solution for a period of time. Thismay form patterned or random networks of graphene.

F. Printing

The graphene network may be transferred from one substrate to another bymeans of a stamp. The stamp may be made from PDMS(Polydimethylsiloxane). The transfer can be aided by gentle heating (upto 100 degrees Celsius, and pressure).

Seventh Embodiment Devices that Incorporate Graphene Networks

Another embodiment of the invention is devices incorporating graphenefilms. Such devices are fabricated differently from existing devicesthat utilize other conductive and transparent films, and these deviceshave new and improved functionality. For example, sensors to detectchemical or biological species can be fabricated where the graphene filmforms one of the conducting channels. Solar cells and light emittingdiodes currently used indium tin oxide as the transparent electrodes.New organic and inorganic solar cells and light emitting diodes can befabricated based on graphene films. Due to the mechanical flexibility ofthe graphene films, such solar cells and light emitting diodes can beflexible rather than rigid. Similarly, touch screens have recurringlifetime issues due to the fact that the indium tin oxide electrodes arebrittle. New touch screens based on graphene films have longerlifetimes. Graphene films can also be incorporated into plasma andliquid crystal displays.

The present invention has been described above with reference topreferred features and embodiments. Those skilled in the art willrecognize, however, that changes and modifications may be made in thesepreferred embodiments without departing from the scope of the presentinvention. These and various other adaptations and combinations of theembodiments disclosed are within the scope of the invention.

1. A method of producing a film, comprising: applying a dispersion comprising graphene flakes and a solvent to a surface of a substrate; and removing the solvent from the dispersion such that the graphene flakes form an interconnected network; wherein the graphene flakes are functionalized; wherein the interconnected network has an optical transparency of at least 50%, and wherein the interconnected network is electrically conductive.
 2. The method of claim 1, further comprising applying a polymer to the interconnected network.
 3. The method of claim 2, wherein the dispersion further comprises a surfactant.
 4. The method of claim 3, wherein the interconnected network has a sheet resistance of less than 50 kOhms/sq.
 5. The method of claim 1, wherein the polymer forms a distinct layer adjacent to the graphene flakes.
 6. The method of claim 1, wherein polymer forms a composite with the graphene flakes.
 7. The method of claim 1, wherein the dispersion further comprises carbon nanotubes.
 8. The method of claim 1, wherein the dispersion further comprises a surfactant.
 9. A method of producing a film, comprising: applying a dispersion comprising graphene flakes and a solvent to a surface of a substrate; and removing the solvent from the dispersion such that the graphene flakes form an interconnected network, wherein the dispersion further comprises a surfactant; wherein the graphene flakes are functionalized; wherein the interconnected network has an optical transparency of at least 80%, and wherein the interconnected network is electrically conductive.
 10. The method of claim 9, wherein the interconnected network has a sheet resistance of less than 2 kOhms/sq.
 11. The method of claim 9, further comprising applying a polymer to the interconnected network.
 12. A method of producing a film, comprising: applying a dispersion comprising graphene flakes and a solvent to a surface of a substrate; removing the solvent from dispersion such that the graphene flakes form an interconnected network; and applying a polymer to the interconnected network; wherein the graphene flakes are functionalized, and wherein the interconnected network has a sheet resistance of less than 50 kOhms/sq and an optical transparency of at least 50%.
 13. The method of claim 12, and wherein the dispersion further comprises a surfactant.
 14. The method of claim 12, further comprising pre-treating the surface to enhance adhesion of the graphene flakes thereto.
 15. The method of claim 12, wherein the dispersion further comprises carbon nanotubes. 